Environmental Biology BIO 110
Dr. Charles Chester


Populations, whether of humans or other organisms, change over time depending on births and deaths as well as migration into and out of an area. Most populations fluctuate mildly over time. Prior to the Industrial Revolution, the human population was stable with minor fluctuations. Populations grew very slowly and growth was apparent only after thousands of years. Since the Industrial Revolution human populations have increased dramatically, growing exponentially with each year and reaching 6 billion by October 1999.

As the population continues to increase, other problems have become worse. All these people need food and resources to live. Where will this material come from? All these people produce wastes. How will we get rid of the waste? Will populations continue to grow exponentially?

There are two opposing arguments concerning the future of the human population.

1. Malthusian Overpopulation: Based on an 18th century English economist who predicted that populations will grow exponentially while food resources would grow arithmetically. Eventually, the population size would be greater than the available resources and the population would crash.

2. Cornucopian Stability: Humans are creative. Any problem that develops as our population grows will spark creative inventions that will enable further growth.

Both arguments have merit, but neither can be completely correct. Malthus predicted that the human population would crash in the 1900ís. This never occurred primarily because of new methods for dealing with waste and new techniques for increasing food supply. On the other hand, the cornucopian viewpoint cannot support endless growth. At some point resources become limited, in spite of our creative abilities.

Demographers are people who study the statistical characteristics of population growth. They look at data for a local area and make predictions about population trends. These scientists most commonly use two types of graphs: the population growth curve and the population profile. In this assignment, you will create these graphs for a county in Alabama.

Population growth curves plot time versus population. The figure below illustrates a population growth curve for Mobile County from 1820 to 1990.

Mobile illustrates a growing population. Over time it has become a large metropolitan area. Notice the decline in growth around the 1870ís and the increased growth in the 1940ís. What occurred during these time periods?


The first part of the assignment will be to construct a population growth curve for a selected county in Alabama. You will use a government document entitled:

Forstall, R.L. 1996. Population of States and Counties of the United States: 1790-1990. Department of Commerce. U.S. Bureau of the Census, Population Division.

In this document you will find historical population data for every county in the United States. Choose a county in Alabama and record the following information below.

Population Growth Data Sheet

County: ____________________________________

First Census (year): ___________________________

Fill in the population size for each decade, beginning with the first.

1800 __________________________

1810 __________________________

1820 __________________________

1830 __________________________

1840 __________________________

1850 __________________________

1860 __________________________

1870 __________________________

1880 __________________________

1890 __________________________

1900 __________________________

1910 __________________________

1920 __________________________

1930 __________________________

1940 __________________________

1950 __________________________

1960 __________________________

1970 __________________________

1980 __________________________

1990 __________________________

Using the above graph as a model, construct a population growth curve for your county on the grid provided in the assignment. Plot the census year on the x-axis and the population on the y-axis.


1. Did the population in your county drop between 1850 and 1870? By how much? What are some reasons that may have explained this decline?








Calculation of Growth Rates

A simple measure of growth rate involves subtracting the birth rate (b) from the death rate (d) and adding migration. Find the appropriate information for your county on the accompanying sheets and enter the values below.

Births 1998-99 ____________________

Deaths 1998-99 ___________________

7/1/98 estimate ___________________

Birth rate is calculated as the number of births divided by the population estimate, as follows:

b = births/98 estimate = __________________________



The death rate is likewise calculated as the number of deaths divided by the population estimate, as follows:

d = deaths/98 estimate = __________________________



Both of these values represent per capita rates (per person), meaning that for every person, b births and d deaths occurred. In human population statistics, birth and death rates are sometimes given as crude rates. Crude birth and death rates are calculated by multiplying the above rates by 1000.

The population growth rate is often expressed as a percentage and can be measured as the difference between birth rate and death rate using the formula below.

Growth rate (g) = (b-d)*100 = __________________________

In addition to births and deaths, population growth rates are also affected by migration, either into a population (immigration) or out of a population (emigration). The U.S. Bureau of Census gives this information as a net international migration into and out of the country and a net domestic migration into and out of a county. Note that these values are positive if the net migration is into a region and negative if the net migration is out of a region. Find the appropriate international and domestic migration from the accompanying sheet. Remember to keep the plus or minus.

Net International Migration ___________________________________

Net Domestic Migration ______________________________________

These must also be converted to rates to be added to the growth rate. This can be accomplished as with the births and deaths above.

International Migration Rate (im) = net international migration/98 estimate = _____________________________________

Domestic Migration Rate (dm) = net domestic migration/98 estimate = _________________________________

We can add these values to our population growth rate by adding the sum of migration as follows:

Growth Rate (g) = ((b-d)+(im + dm))*100 = _______________________


1. Is your population growing or shrinking?

2. How does migration affect the growth rate?





Calculation of Doubling Time

Doubling time is the time in years it takes a population to double in size. It is easily estimated by dividing 70 by the population growth rate, as follows:

Doubling Time (td) = 70/g = ____________________

Population Profiles

Not every member of a population is likely to reproduce. Populations include juveniles, and post-reproductive individuals in addition to reproductive individuals. We can map this by making a population profile graph.

The graph above illustrates the population profile for Mobile County. Age is divided into increments of 4 years and appears on the y-axis. On the x-axis are percentages of each age class to the total population. Note that males and females appear on two separate columns. With this type of graph, one can observe the effects of mortality as well as migration. Note the relatively smaller percentage of 20 to 30 years olds as well as the slight bulge in the 40 years olds. These represent migration and the baby boom.

Using the information provided, construct a population profile for your county on the accompanying grid.


1. How does the population change with age?



2. What trends in population growth does this graph suggest to you?